Stu's right. It was all a scam and Dia was signed after he got his mate to phone up Graeme Souness and pretend to be George Weah and say that Dia was his cousin and a Senegalese international who had scored twice in his last international and that they had played together at PSG. All of which was a complete lie and Dia was just some amatuer footballer, but none-the-less he was signed on trail by Southampton.
His big chance came when Matt Le Tissier was injured after 32 minutes of a match against Everton and Dia came on to replace him. It soon became clear, however, that Dia was not the player that Souness thought he was, or indeed, a player at all! Having chased the ball around for 53 minutes without actually getting involved at all, he was himself substituted and promptly released from the club.
It later transpired that it was not George Weah on the phone but a mate of Dia's! Souness' reaction to this news was 'Well, he sounded like George Weah!' If I remember correct he then disappeared, before appearing for Gateshead breifly, and opening a Bangladeshi restraunt called Kuti's Brasserie, in Southampton.